Frequency modulation system



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. FREQUENCY MODULATION SYSTEM Filed Dc. 26, 19414 BUE/1'? Ri.' -f E Ji 1 '-[AMBJ AME 1, 17150.,r Am,

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- .112 cme. m? l@ 'fue '117 Patented, Oct. 9, 1945 UNITED STATES orifice. I

FnEQUENoYririonsysrun Raymond M; Wilmotte, Washington, D.' C'.

Application j 11 claims. This invention relates to multi-signal radio transmission', and, particularly, t0 a system adapted-to receive, selectively, frequency-modulation signals'in the same-frequency Iband. f

'This application relates to subject matter similar to thatV disclosed in my copending application, Serial Number 424,521, filed December26, 1941. While the separation of two signals is well known in the radio art, .the existing methods f separate the signals in accordance 4 quency difference between them. According to the present invention, however, separation is ob than that-of the other, both signals covering a part of the same frequency band.

Yet another object of my invention is to provide a system applicable to presentday frequency modulation receivers and utilizing, for the most pant, components similar lto those found in thel receivers to which it is to be applied.

In a `frequency modulation receiving system as now known, when two signals of different ainplitude covering a common frequency band are simultaneously received, one of the successive steps in reception is to treat the signals so as to increase the ratio, in amplitude, between the stronger and weaker signals. A broadobject of this invention is to utilize the difference in ratio, between the signals before and after treatment, as a basis, criterion, factor, or the like, in the reception. Thisutilization may be manifest in any one of a' plurality of manners, the examples illustrated herein lconstituting but one of the many applications 0f the invention. Y Other objectives of the invention will loecome evident 'to those'familiar with the art from the following specification and drawing which illustrates, in block diagram, a circuit for separating two frequency-modmation signals of different carrier intensity. In-the drawing: Y

- Figure 1- represents in blockdiagram a-sinrple circuit 4according to this invention to receive two signals of unequal intensity. Figure '2 represents in block diagram `a circuit similar 't-o that shown in Figurefl', but in which a degenerative feedback circuit-'lis used for Vpur-V .posesrexplai'nedhereinaften 1' f with some fre-'v December 26, 19.41, Seria-l No.. 424,522

Figure A3.shows a modification of a portion of 'i tively,l impinge. upon receiving antenna I, the. signal A being'strongerthan iB.A Both signals are amplified: through .radio kfrequency amplifier 2, the frequency isthen `changed to. .an intermediate frequency by a superheterodyne .circuii-Icomprsing detector 13 and oscillator 4, the .signalsthereafter being further amplified in an intermediate frequency amplifier 5 and .passed through ay limiting stage v6. rAfter the limiting stage 6,@the ratio offthe strengthfof-signal .A to that of signal B is greaterfthant. was at the .output of :the :receiver. If the signal were then passed `through a discriminator iban` audio frequency .amplifier IB anda loud .speaker I.I,.the audio .output would consist `mainly-of. the audio modulation of signal A. In 'the present state of the art, .the sound output 4can be made substantially independent of signal B if the :ratio ofY the strength of signal A to' that of "B, at the 'input of the receiver, 'be of the orderof two to one or more.

However, in accordance withthis invention, the signal, which consists mostly-of signal A, is attenuated atthe output of the-limiter stage by an attenuator, ,such as a potentiometer "l and variable Ltap. .8. Additionally, a portion of the input tol intermediate Afrequency amplifier 5, is passed through a l:buier'stage I2 to,v prevent feed-back. 'I'heutput 0f .buffer ,stage l2 is vthen combined at`18 with the output of attenuator ,to form the input of an intermediate frequency amplifier I3.

Theoutput of attenuator 1, which consists mainly of signal A is .combined with the .output .of `buffer stage I2, which-.consists of signals .A and B, with A predominating vso that the vA signals from the attenuator land buierIZ .substantially cancel one anotlaxer...V Theresultant inputfto intermediate frequency-ampliiier I3 is. then dominated by the remainingV Bsi-gnal. It is found preferable to use a tunedn-circu-it Vas :indicated at 38. If desired, the outputs-ot attenuator 1 and `buffer stage. I2 may be adjusted inphase and amplitude relative to oner :another -bfylphase control-means included in -318 .so as -to ins-,ure substantial cancellation 4of theA signals. The intermediate frequency amplifier i3. Tis-connected in the fusualmanner to a limiter/I4, discriminatorjli, audio amplifier 16,

' and loud speaker Il. :thatenough of the signal is cancelled :priori intermediate amplier t3 .so that its' lstrength is .onelhalf or less that of B, according to present .day technique,

the output of the speaker I1 will be substantially independent of signal A.

It is not necessary to have separate discriminators 9 and I5, audio amplifiers I0 and I6, and loud speakers I I and I1, unless both signals A and B are to be received simultaneously. A single discriminator, audio amplifier and loud speaker are sufficient. By means of a switch I8 the out-v put of the limiters 6 or I4 can be connected to this discriminator, audio amplifier and loud speaker as shown in Figure 3, softhateither signal A or `B may be received at will by operating the switch.

sities are selectively detected and separated by amplitude discrimination;

The two components of the signals! which are shown combining at I8 prior to being fed into amplifier I3 need not be picked out at the' points shown. These 4com'ponentsrnust .be of the Same frequency and of proper phase and amplitude.,

One-must be taken .prior to the limiter oran equivalentcircuit'which has the property of increasing the eectiv'e 'ratiooffsignals A and B andthe other after. this limiter circuit. The first component can Atherefore 'be taken at any convenient point within the amplier 5 or even at the input ofthe limiter 6..

In the circuitdescribedabove, the signal A is substantially balanced out. by combining algebraically with a portionof Vthe output of limiter 6. Another circuit which can be lused for the same purpose is4 shown. inV Figure 2, in which,

elements corresponding to those described in the first embodiment .are designated bythe `same reference numeralsV plus III.` This circuitissimav ilar to thatof Figure 1 with the exception that one of the components ofthe signal forming the input to amplifier II3 is taken from Vthe input of the limiter |06, (o'r if desired, some part of the,

age to amplifier I05,'the degeneration of signal A will be greater than that of signal B, thus tending to make the signal at the output of amplier 'contain the signals A and B in more nearly equal proportion than at the input of amplifier |05. 'The greater the feed-back, the more nearly equal will signals A and Bv become at the outputof the amplier'l05 and, incidentally, the `more stable will the amplification be, thus helping to make thebalancingout of signal Aat the input of amplier- I I3 more reliable.

It will be'evident that the invention is not limited to the detection of the weaker of two signals. A third signal C, for instance, weaker than` B, could be detected by combining a por-- tionof the output of limiter II4,` with the input of amplifier |.I'3' so as lto obtainfor the input or more programs on the same frequency band. Programs could be used to modulate carriers of different amplitude, the modulated signals then being fed preferably to the same antenna. The received signals would be of the same relative intensity as that of the transmitting antenna. The received signals could then be individually separated by means of a receiving circuit such as that described above.

The invention described above is not limite to the specific disclosure, but, on the contrary, embraces the full scope of equivalents and suby j ,stitutions within the scope of the following Thus, without making use of Vfrequency Vdiffs-v crimination, signals A and B of different inten-- claims,-

.1. Inafrequency modulation radio receiving vsystem,including radio frequency means for re- 'ceiving andA amplifying two signals of different to another channel a signal in-which the signal.

Cwas of dominant strength. u

Whilethe-inventionmay bei used to receive separately randomV signals 'of different intensity,`

'it may rbe used Lalsoto transmit and 'receive-two discriminaton, a second circuit connecting theg inputwof saidldiscriminator to the output of said limiter;` V,and audio output ,means consecutively.

connected Vtothe output of said discriminator, an attenuatorfconnectedj across vsaid second circuit for attenuating a portion of the output of said,V

limiter, abuifer connected to said rst circuit,

means forV combining the outputs of said attenu ator and said buffer so; as'to nullify the stronger signalA component in the output of said buffer withthe stronger signal componentpin the output of saidv attenuator, whereby a resultant is pro-l duced'in ,which the weaker'of the signals originally' received dominates the stronger, and a limiter, discriminator, and audio output means consecutively connected to the output of the lastmentioned means.

y 2.7 In fthe 1 combination ,claimed yin vclaim 1, means'` in vthe output ofV saidlattenuator for adjusting the phase-andy amplitude ofthe attenue ated portion of 'the output ofthe first-mentioned i limiter. g, e A 3. 1n ai frequency modulation radio receiving system, including an intermediate frequency'amplifien stage'and,a limiterI stage, means for detercting' a" iirstl -frequency modulated stronger signal and sa second weaker signalY coyering over,-

.lapping interfering portionsof the same ifrequency band comprising, degenerative Vfeed-back' means for g injecting a. portion of the output ofl said limiter stagef into'fsaid'fmtermediaterfree quency stage, means for f' combiningppprtionsvof the input and output;irespectively, y of 'said 4 intere,

mediate frequency amplifier 'stage'. s0 as toproduce a resultantdominated 'by said second signal and means Yresponsive.-to- 'a'V dominant signal for respectively utilizing: themesultantoutputs of said limiter and saidjlastfnamedmeanspii method of 1 receiving radio 4intelligence comprising receivingra 'combination' of a first'and a fsecond signal rcovering-Qverlappingi interfering partsv'of -thesamey frequency bandythe stronger signal being frequencyvmodulatedgsaid jfirst signalr being ;,Si'1f0r1geryrthan; :'sadnzsecond signal, Changing; the combination of;.said-ifirst-and4 secr. Ondqsignals by increasing the. ratqxi of. the ,am.- plitude of the modulatonbetweenathef nrstiand;

, quency modulated, comprising a rst means for receiving the first and second signals, second means for changing the ratio of the amplitude of the modulation between the `first and second signals, circuit means for combining the output of said first and second means for balancing out from the output of said first means a substantial portion of said second signal for deriving a resultant output in which the strength of the first signal relative to the second is increased,l

and means for utilizing the resultant output.

6. The combination claimed in claim 5, and

vmeans in said circuit means for adjusting the phase and amplitude of the output of said secy ond means relative to that of said first means.

l'7. The combination claimed in claim 5 and the last named means including means for increasing the ratio between said second signal and Said first signal.

8. In a frequency modulated radio receiving system, means for detecting both of two signals of different amplitude covering overlapping interfering portions of the same frequency band, the stronger signal being frequency modulated,

Vcomprising a first means for receiving the first,

stronger signal and a second, weaker signal, second means for increasing the ratio of the am plitude of the modulation between said rst and second signals, means, including audio output means, responsive to said first signal connected to the output of'said second means, circuit means for combining a portion of the output of said second means with a portion of the output of said first means whereby the major portions of the first signal, in said circuit means, cancel one another so as to produce a resultant in the output of said circuit means in which said second signal is the stronger, and third means connected to the output of said circuit means for increasing the ratio of the amplitude of the modulation of said second signal relative to the first, and means, including audio output means, responsive to the stronger, second signal connected to the output of said third means.

9. In a frequency modulated radio receiving system, means for selectively detecting either of first andrsecond signals of different amplitude covering overlapping interfering portions of the samek frequency band, the first signal vbeing y stronger and frequency modulated, comprising first means for receiving the first stronger signal and second, weaker signal, means for amplifying said signals, second means for changing the ratio of the amplitude of the modulation between said first and second signals, attenuating means for attenuating a portion of the output of said second means, circuit means for combining said attenuated portion with a portion of the output of said first means whereby the major portions of the first signal in said circuit means cancel one another so as to produce a resultant in the output of said circuit means in which the second signal is the stronger, third means connected to the output of said circuit means for increasing the ratio of the amplitude of the modulation of said second signal relative to the first, fourth means, including` audio output means responsive to the stronger of the two signals, and switch means for selectively connecting one of said second and third means with said fourth means while disconnecting the other.

10. A method of receiving radio intelligence from two frequency modulated signals covering overlapping interfering parts of the same frequency band, the stronger signal being frequency modulated, comprising simultaneously receiving the two signals, deriving a rst resultant wherein the signals have one ratio, increasing the ratio between the two signals, deriving a second resultant wherein the signals have a second ratio, reacting said first and second resultants to derive a third resultant, and utilizing said third resultant for the detection of said4 intelligence.

11. A method of receiving radio intelligence from two frequency modulated signals covering overlapping interfering parts of the same frequency band, the stronger signal being frequency modulated, comprising simultaneously receiving the two signals of different intensity, deriving a first resultant wherein the signals have one ratio of amplitude of modulation relative to each other, changing the ratio of the amplitude of ymodulation between the two signals, deriving a second resultant wherein the signals have a second ratio of amplitude of modulation relative t0 each other, reacting said first and second resultants to derive a third resultant, and utilizing said third resultant for the detection of said intelligence.

RAYMOND M. WILMOTTE. 

